1. Field of the Invention
The present invention relates to a laser irradiating torch device used for applying a corrosion resistant coating and other like coatings on the internal surfaces of a small diameter piping.
2. Technical Background
Lasers have been used for fabrication, such as cutting and making holes in metallic materials, and laser fabrication devices of various kinds are on the market. In the field of nuclear power generation, to improve corrosion resistance performance in nuclear facility, laser devices have been used to apply a corrosion resistant metallic coating on internal surfaces of piping circuits.
Application of a coating in such cases generally involves painting a material containing a binder and metallic powder of corrosion resistant metals on the interior surface of the piping, which is followed by laser heating to vaporize the binder and melt the metallic powder to form a coating layer. By adopting such a procedure, it is necessary to heat only the surface layer of the piping interior, thus avoiding possible deformation and property changes which could be caused by heating the entire piping component.
However, the piping in nuclear facilities tends to be small diameter (30 to 40 mm diameter) and long length (about 5 m), and the existing laser technology does not readily allow insertion of a laser device to such narrow diameter piping. In particular, application of corrosion resistant coating to the deep recesses of the piping by laser technique was extremely difficult.
As an example of such laser coatings, a laser irradiating torch device is disclosed in a Japanese Patent Application, First Publication H3-99787, for internal surfaces of small diameter piping. This laser irradiating torch device is provided with a cylindrical torch body which is attached to the tip of optical fiber which transmits a laser beam. At the tip of the torch body is disposed a torch head of approximately the same outside diameter as the torch body. The outside diameters of the torch body and the torch head are smaller than the inside diameter of the small diameter piping into which the torch device is to be inserted. In the interior of the torch body, there is disposed a set of lenses to focus the laser beam transmitted by optical fiber. In the interior of the torch head, there is disposed a reflecting mirror which reflects the laser beam which passes through the aforementioned set of lenses. On the sidewalls of the torch head, there is disposed a radially directed small diameter hole. The laser beam reflected by the reflecting mirror is radiated to the outside of the torch head through the small hole.
The laser beam from the torch head is directed to every part of the inner surface of small diameter piping in the following manner. A torch device is inserted into a small diameter hole, and in this condition, the torch body can be either fully rotated around its own axis or linearly translated along the longitudinal direction. While the torch body is either rotated or translated, the laser beam is transmitted through the optical fiber. As a result, the laser is radiated to the various locations, thereby every part in the inner surface of piping can be irradiated with the laser beam.
However, there are following problems associated with the laser irradiating torch device which works as described above.
(A) When it is necessary to irradiate the inner surface of the piping, the torch device is revolved while being moved along longitudinally, therefore it is generally difficult to coincide the centers of rotation of the torch device with that of the piping. It follows that the variations, in the distance between the radiation hole at the torch head to the inner surface of piping, can occur at different locations along a pipe, resulting in non-uniformity in the power of laser beam delivered to the various locations in the circumferential and longitudinal directions. PA1 (B) The rotation of the torch device itself can lead to the application of twisting torque to the optical fiber connecting the laser beam source and the torch device. PA1 (C) The reflection mirror which is placed at the deep end of the small diameter piping is not cooled and it is subject to thermal degradation, The frequent periodic maintenance required represents an increase in the operational steps, PA1 (D) The laser beams scattered back into the optical fiber from the inner surface of the piping can lead to degradation of the performance of the optical fiber,